In a magnetic disk drive, servo data for positioning a magnetic head (hereafter simply referred to as a “head”) to a target position on a magnetic disk (hereafter simply referred to as a “disk”) is written on the disk itself. Specifically, the servo data is written in servo regions circumferentially arranged on the disk in a discrete manner at particular intervals. In order to position the head to a target position with high precision based on the servo data, a track where servo data is written desirably has a breadthways center, or a track center, which describes an ideal perfect circle.
However, it is general that what is described by the track center of a track on a disk is distorted and is deviated from a perfect circle. This is mainly caused by a wobble of the axis of rotation of a spindle motor which rotates the disk, i.e., a wobble in synchronization with the rotation of the disk. When what is described by the track center of a track is distorted, servo data will include a position error resulting from the distortion concerned, or, a position error resulting from a wobble which occurs in synchronization with the rotation of the disk and is called repeatable runout (RRO).
A magnetic disk drive executes RRO learning of a track center at a particular track, obtains RRO correction data, and writes the obtained RRO correction data in the track center of the particular track. When the magnetic disk drive writes the RRO correction data in the particular track, it first writes servo data, and then writes RRO correction data for correcting a position error resulting from RRO.
When the magnetic disk drive positions the head on the particular track, it reads the written RRO correction data, obtains an RRO correction amount from the read RRO correction data, and makes an adjustment to the position where the head is located using the obtained RRO correction amount.
When the RRO correction amount obtained by RRO learning executed while the head is at the track center can be read even when the head is located at a position distant from the track center by a particular distance (hereafter referred to as an “offset position”), the magnetic disk drive corrects the head position using the obtained RRO correction amount. In such a case, an actually corrected amount may be different from a proper RRO correction amount which should have been corrected. Accordingly, a servo positioning error (Repeatable Position Error: RPE) may be large when the head is at the offset position. Therefore, unlike the case where the head is at a track center, the magnetic disk drive may be unable to execute suitable RRO correction when the head is at the offset position.
When the head is at an offset position and RPE is large, it is highly possible that the head will leap out of a region where data writing is permitted. As a result, retries may occur, which causes rotational delay. Write performance will fall. Moreover, there is a possibility that adjacent data may be erased. Accordingly, it is hardly possible to narrow a data track pitch.